Metal-ceramic structure with intermediate high temperature reaction barrier layer

ABSTRACT

A Si--SiC ceramic layer is bonded to a non-porous SiC substrate with the Si etched from the layer to form a relatively porous surface on the otherwise non-porous high strength SiC substrate. A quartz layer is softened by heating and forced into the pores of the porous layer to form a mechanical bond to the SiC substrate. A refractory metal layer is bonded to the quartz layer to complete the joint. A refractory metal support component is then bonded to the refractory layer whereby the quartz serves as a high strength, high temperature reaction barrier between the metal of the refractory layer and the silicon of the SiC substrate.

This invention relates to structures for joining metal to ceramicsubstrates.

The joining of a metal structure to a ceramic substrate is of presentwidespread interest. Ceramic materials provide chemical inertness tocorrosive or oxidative environments, strengthening and stiffness atambient and elevated temperatures, and other properties not exhibited byother materials. The metal provides complimentary high strengthproperties so the ceramic-metal system produces technological advantagesnot otherwise possible with the ceramic or metal alone.

One particular problem involves a high temperature-high stressenvironment. In certain heat exchanger applications, the tubes of theheat exchanger are required to withstand high temperatures, e.g., in therange of 750°-1000° C. and high internal pressures, e.g., 1500 psi. Itis recognized that ceramic materials are better than metals for thispurpose. High temperature fluids flowing through the tubes of a heatexchanger tend to cause excessive oxidation if the tubes were metal.Even high temperature refractory metals heavily oxidize at temperaturesin the temperature range mentioned. Therefore, ceramic tubes are moredesirable.

However, a problem arises using ceramic tubes. Not all ceramics arecapable of withstanding temperatures in the range of 750°-1000° C. Thepresent inventors recognize that silicon based ceramics can withstandsuch temperatures. One solution, therefore, is to secure silicon basedceramic tubes to refractory metal support structures. But, the effectsof exposure of metal-ceramic direct joints for relatively long periodsat temperatures in excess of 700° C. in an oxidizing environment isknown to seriously effect the quality of any such bond. Even in thepresence of an inert external atmosphere, at temperatures above 700° C.,it is known that severe metal-ceramic reactions take place betweensilicon based structural ceramics, such as silicon carbide and siliconnitride, and most metals. These reactions produce silicides whichseriously weaken the bonded joint. Silicides are relatively brittle andtherefore tend to fail under the stresses induced by the high pressuresof the system.

The joining of metals to ceramics, however, is the subject of long-termongoing studies toward the solution of combining these materials forparticular applications. The joining of ceramic-metal systems, forexample, is discussed in the Encyclopedia of Materials Science andEngineering, Vol. 4, 1986, pgs. 2463-2467, in an article entitled"Joining of Ceramic-Metal Systems: General Survey" by V. A. Greenhut andin an article entitled "Joining of Ceramic-Metal Systems: Procedures andMicrostructures" by J.T. Klomp at pages 2467-2475. While these articlesaddress generally the problems of joining metal or glass to ceramicsubstrates, they do not deal with the high temperature, high stressenvironment problem to which the present invention is directed, andparticularly, how to join a metal to a ceramic capable of withstandinghigh temperatures in the range of 750°-1000° C. Still other discussionsof ceramic/metal joints for structural applications are in an articleentitled "Ceramic/Metal Joining for Structural Applications" by Nicholaset al. Material Science and Technology, September 1985, Vol. 1, pgs.657-665. This article also does not address the high temperature-highstress problem faced by the present inventors.

In the encyclopedia article by Greenhut, for example, the bondingmechanisms discussed at page 464 discuss the bonding of metal to ceramicor glass to ceramic. The article discusses mechanical bonding where aliquid metal or glass can penetrate pores or cavities in the solid toprovide further mechanical bond because of the interlocking nature ofthe structure. Ceramic chemical bonding is also discussed wherein metalto ceramic chemical bonding is difficult. Glass can be joined to anoxide coating of metal. However, the glass referred to, in any case, isa low temperature type normally not capable of withstanding the hightemperature environment in a heat exchanger of the type discussed above.The metal-silicide problem is not discussed at all.

At page 2465, the article states that certain refractory metals aresuitable for use in ceramic joints, but only where their oxidationresistance does not create a problem. Obviously, temperatures of750°-1000° C. create such a problem. Generally, the article discussesseveral metal-ceramic bonding methods in paragraph 5 at pages 2465 etseq. These processes are used to produce a metal coating suitable formetal brazing methods. This is stated to be the most common method forproducing a ceramic-metal joint. A brazed joint, however, can notwithstand the 750°-1000° C. range. Various other metallizing techniquesare discussed but all are implicit in creating a metallic coatingsuitable for subsequent brazing. For example, at page 2466, the articlediscusses using ceramic glazes and glass frits with a low softeningtemperature to join ceramics to each other and to metals. The article issilent however, in what to do in high temperature environments. Thearticle by Klomp suffers from similar drawbacks.

In the article by Nicholas et al., at page 664, a discussion of mostjoints is made in which the joints are fabricated using intermediatebonding agents. However, these intermediate bonding agents employapplications of low temperature glass frits and brazing techniques whichare in widespread use. Glass frits are a material which has a softeningtemperature below 600° C. and therefore is not capable of withstandingthe relatively high temperatures of 1000° C. Thus, the problem to whichthe present invention is directed involves high temperature, highpressure applications employing a structural ceramic, such as siliconcarbide or silicon nitride, and bonding a metal to that ceramic suchthat the bond does not deteriorate due to differences in coefficients ofthermal expansion (CTE) or chemical reactions which normally occur atthe ceramic-metal interface. The present inventors recognize a need fora metal-ceramic structure which is capable of withstanding relativelyhigh temperatures and high pressures without subsequent mechanical orchemical reaction failures of the joint.

A structure in accordance with the present invention comprises forming asilicon based ceramic substrate and securing a layer of glass to thesubstrate wherein the glass has a softening temperature of at leastabout 750° C. The glass serves as a barrier to chemical reaction betweena metal member secured thereto and the silicon in the substrate. Arefractory metal member is bonded to the layer of glass.

In one embodiment the metal member is a refractory material and theglass is quartz, for example, fused silica. This structure can withstandthe stresses produced by relatively high pressure differentials, forexample, pressure differentials of over 1500 psi and relatively hightemperatures of 750° C. and greater without chemical or mechanicalfailure of the joint.

IN THE DRAWING:

FIG. 1 is a sectional view of a high temperature metal-ceramic structurein accordance with one embodiment of the present invention; and

FIG. 2 is a sectional elevation view of the metal-ceramic structureaccording to the embodiment of FIG. 1 in which a ceramic tube is securedto a support.

In FIG. 1, a portion of a structural ceramic component 10 is shown whichcan withstand temperature cycling, for example, between 0° C.-1000° C.without thermal shock failure or without thermal stress failure of thejoint 12. The joint 12 joins a ceramic substrate 14 to a metal element16. At elevated temperatures, for example, at or above about 750° C.and, preferably at 800° C., the joint 12 precludes reaction of the metalelement 16 to the material of the substrate 14 and withstands tensilestresses induced by pressure differentials of about 1500 psi andatmospheric pressure. The substrate 14 is a silicon-based ceramic whichmay be, for example, silicon carbide or silicon nitride. These materialsare capable of withstanding elevated temperatures at or above 750° C.without deleterious effects. For example, alumina based ceramics tend tosoften at temperatures at or above 800° C. such that the modulous ofelasticity decreases significantly. The ceramic substrate 14 preferablyis one of relatively low porosity for high strength application. Asilicon carbide material, for example, exhibits a tensile stress ofabout 20,000 psi. Silicon carbide material, for example, may befabricated by sintering a powder with hot isostatic pressing to removethe porosity required to produce a high strength material. The lowporosity of the substrate 14 thus precludes the mechanical bonding of ametal element, such as element 16, directly to the substrate asdiscussed in the encyclopedia article mentioned in the introductoryportion. The mechanical bonding referred to in these articles indicatesthat liquid metal or glass can be attached to a roughened surface bycausing the fluid to penetrate pores or cavities in the solid to providea mechanical bond via the interlocking nature of the structure.

Further, the composition of the substrate 14 is limited by the need tohave the substrate retain its characteristics at temperatures above 750°C. For example, the Nicholas et al. article mentioned in theintroductory portion, at page 658, discusses the bonding of glasses andmetals to ceramics by fusion bonding. As discussed therein, the range ofmaterials which fusion bonding is useful is limited. As furtherdiscussed therein, there should be ideally a close match of meltingpoints and thermal contraction characteristics of not only the metal andthe ceramic, but also the complex material formed in the well pool. Thearticle states that this similarity is rarely achievable in practice andthat some ceramics such as BN, SiC, Si₃ N₄, sublime or decompose beforemelting while others such as MgO vaporize rapidly when molten. Further,on cooling, disruptive phase transformations may occur in certainceramics.

Different properties of certain ceramics and metals are disclosed inTable 1 in the Nicholas et al. article. While the various ceramicsdisclosed in Table 1 have relatively high melting points, a problemremains with the sublimation or decomposition of certain of the ceramicsat temperatures significantly lower than their melting points. For thisreason, substrate 14, FIG. 1, to prevent sublimation and decompositionat temperatures at or above about 750° C. of silicon based substrateswhich tend to withstand structural stresses and also are capable ofwithstanding temperatures at or above about 750° C. without sublimation.As mentioned in the introductory portion, a problem with the siliconbased substrate however is the potential capability of forming silicideswhen a metal element, such as element 16, is directly bonded to such asubstrate. For this reason joint 12 is provided structure which servesas a reaction barrier to the reaction of the metal of element 16 to thesilicon of substrate 14 to prevent the formation of silicides and thusthe weakening of the joint forming the bond between the two materials.

Joint 12 comprises a Si--SiC substrate layer 18 which is bonded tosurface 20 of the silicon based substrate 14. The Si--SiC layer 18 canbe bonded to the substrate 14 using known technology to provide a highstrength joint due to the fact that the layer 18 and the substrate 14are substantially similar materials, i.e., SiC.

The purpose of bonding layer 18 which may be, for example, a millimeteror less in thickness, to the substrate 14 is to provide a porous surfaceto the relatively non-porous substrate 14. To create a porous surface,the silicon in the Si--SiC layer 18 is etched away leaving a poroussubstrate in the regions of the layer formerly occupied by the Simaterial. The etching of a silicon-based ceramic material is disclosedin more detail in U.S. Pat. No. 4,109,050 assigned to the assignee ofthe present invention and incorporated by reference herein. As discussedin the patent, etching solutions can be used which include, for example,mixtures of hydrochloric and nitric acid. The silicon-based ceramic istreated with the etching to effect the removal of at least 0.001 (0.025mm) inch to 0.010 (0.25 mm) inches of the silicon from the silicon-basedceramic layer 18. After the etching has been rinsed from the surface ofthe silicon-based ceramic, the resultant layer 18 is a roughenedmaterial whose pores are significantly larger than the pores of thesubstrate 14 which is not etched. The etching has no effect on thesilicon in the substrate 14 and only removes the excess silicon portionof the Si--SiC layer 18. This produces a roughened porous surface 22 onthe composite structure formed by substrate 14 and layer 18.

A layer of fused silica 24, more commonly referred to as quartz orvitreous silica, is heated to its softening temperature, for example,about 1670° C. Layer 24 is amorphous rather than crystalline. Beingamorphous, the layer 24 tends to gradually soften as the temperatureincreases rather than have a localized melting point as occurs with acrystalline structure. The fused silica has the properties shown inTable 1.

                  TABLE 1                                                         ______________________________________                                        T.sub.softening = 1670° C.                                                            (can be made to flow)                                          T.sub.set = 1310° C.                                                                  (solid behavior below this                                                    temperature)                                                   Tg = 1150° C.                                                                         (no time dependent behavior below                                             this temperature)                                              ∝ = 5.5 × 10.sup.-7 /°C.                                  ______________________________________                                    

The softened heated fused silica layer 24 is then compressed against theroughened surface 22 of the etched layer 18. The softened fused silicaflows into the interstices of the pores of the etched layer 18 and formsan interlocking bond therein somewhat similar to the procedure describedin the aforementioned encyclopedia article by Greenhut. The layer 24 mayhave a thickness in the range of about 1-2 mm. The depth of the poresinto surface 22, may be on the order of about 0.001 to 0.010 inches(0.025 mm to 0.25 mm).

The composite structure comprising the substrate 14, layer 18, and layer24 comprises a layer of fused silica 24 mechanically secured to thesubstrate 14 via the layer 18. A metal layer 26 of refractory metalselected from the group consisting of molybdenum, tungsten, titanium andtantalium is bonded to the surface 28 of the layer 24. To bond therefractory layer 26 to the layer 24 requires oxidation of the interfacesurface of the metal layer 26 so that there is good wetting between thefused silica and metal surfaces. This procedure is discussed in moredetail in the aforementioned Encyclopedia and Nicholas articles, whichare incorporated by reference herein. As stated in the Encyclopediaarticle at page 2463, in glass-metal joining, it is common topre-oxidize the metal. The resulting oxide layer is compatible withfluid glass, may lower the solid-liquid interfacial energy and therebypromote wetting.

The resulting joint 12 is relatively impervious to wide temperaturefluctuations, for example 0° to 1000° C., and can withstand sudden widefluctuations in temperature within that range without fracture orotherwise weakening the connection of the metal layer 26 to thesubstrate 14. One of the requirements of the joint 12 is that itwithstand not only repetitive thermal cycling but thermal shock at whichthe temperature shifts radically and rapidly in the desired range. Theelement 16, which may be a refractory metal, may comprise other metalsas well. Element 16 is fusion or otherwise bonded to the layer 26 atinterface 30. Metal-to-metal bonding such as at interface 30 is knownand need not be discussed further herein.

While a fused silica layer 24 is illustrated herein, by way of example,other inorganic glasses may be used to form a suitable barrier layer aslong as they meet the requirement of not reacting at high temperatureswith the ceramic substrate 14 or form undesirable silicides with thesubstrate 14. Infiltration of the glass layer 24 into the pores of layer18 may be accelerated by producing a temperature gradient in thesubstrate 14-layer 18 composite structure to further assist the flowingof the layer 24 more deeply into the interstices of the pores of layer18. After the layer 24 is cooled, the infiltrated fused silica in theceramic pores forms an effective mechanical grip which is not degradedby adverse chemica reaction. The fused silica layer 24 after flowinginto layer 18 is a graded layer due to the non-homogeneous ceramic-fusedsilica mixture in the porous region. Thus the mechanical properties inthe layer 18 will be an average of the fused silica properties and theceramic properties of the substrate 14. Thus, a gradation in materialproperties exist in proportion to the fused silica--ceramic ratio whichwill vary from pure ceramic at the ceramic interface at surface 20 topure fused silica at the fused silica-metal interface at surface 28.

Assuming the layer 26 is a molybdenum alloy, the CTE of such alloysmatch quite closely with the layer 24 to minimize stress failure due tothermal shock and differences in CTE in the different materials. TableII below gives the various CTE's for different materials.

                  TABLE II                                                        ______________________________________                                        MATERIAL        CTE (∝)                                                ______________________________________                                        Fused Silica    5.5 × 10.sup.-7 /°C.                             Borosilicate    40 × 10.sup.-7 /°C.                              Soda Lime Silicate                                                                            95 × 10.sup.-7 /°C.                              SiC             40 × 10.sup.-7 /°C.                              Molybdenum      56 × 10.sup.-7 /°C.                              Tantalum        65 × 10.sup.-7 /°C.                              Tungsten        45 × 10.sup.-7 /°C.                              Titanium        94 × 10.sup.-7 /°C.                              ______________________________________                                    

As seen from Table II, there is a variation in a range of 5.5×10⁻⁷ /° C.for fused silica and 94×10⁻⁷ /° C. for titanium. This is to be comparedto a variation of about 50×10⁻⁷ /° C. between the fused silica andmolybdenum and about 35×10⁻⁷ /° C. between fused silica and SiC. It isbelieved that the range of about 50×10⁻⁷ /° C. is the maximum acceptablerange of α between the different layers. With regard to titanium, anintermediate material, such as molybdenum, would be used between thetitanium and the fused silica. Also if the layer of titanium is madesufficiently thin, it could be made to yield in response to thermalexpansion differences. Due to the extreme temperature variations towhich the present structure may be exposed, for example, in the range 0°C.-1000° C., closely matching the CTE's of the different materials isimportant in order to preclude stress failure of the different layers attheir interfaces due to different thermal expansions and contractions atthe different temperatures.

The fused silica in certain embodiments, may be bonded to a metal with arelatively high coefficient of thermal expansion such as the titaniumwhere in this case, the titanium would surround the glass in such a waythat the glass is subjected to compression at room temperature. That is,when the temperatures are increased, the greater expansion of thetitanium is utilized to release the compression forces on the innerglass layer so that negligible tension is present at the glass-titaniuminterface. In other words, at the elevated temperature, there isnegligible stresses exhibited between the titanium and glass layer sothat when the titanium cools it tends to shrink more than the glass,placing the glass in compression. The stress state in glass-metal bonds,as is known, can be dramatically altered by taking advantage of theviscoelastic nature of the glass. For example, various annealingtechniques are available for readjusting or relieving adverse stressdistributions in a joint such as joint 12. The viscoelasco behavior mayalso be used to enhance the high temperature reactions of the joint 12since the glass layer at elevated temperatures might, due to softening,serve as a somewhat damping or cushioning structure relative to shockand vibrations. Also, the porous layer 18 may also serve as a cushion orshock absorber structure.

In FIG. 2, an embodiment of the structure of FIG. 1 is illustrated inwhich a ceramic tube 200 is secured to a refractory metal fitting 202 ina high temperature, high pressure heat exchanger. The tube 200 comprisesmaterials similar to the materials of substrate 14, FIG. 1. Fitting 202may be a refractory metal or other metal element in accordance with agiven implementation. When the environment of the structure of FIG. 2 issubjected to temperature variations of about 1000° C., the fitting 202preferably is a refractory metal. Above 1000° C., even refractory metalsheavily oxidize and become unsuitable. The joint 204 comprises astructure similar to the joint 12, FIG. 1, in which a porous hightemperature-structural ceramic layer 206 is bonded to the ceramic tube200 and a glass layer 208 such as fused silica is secured to the ceramicporous layer 206. The fitting 202 may be directly bonded to the glasslayer 208 or may be bonded to the layer 208 via an intermediaterelatively thin refractory metal layer such as layer 26, FIG. 1, notshown in FIG. 2. The fitting 202 may then be secured by welding, bondingor other mechanical means to a support structure 210.

The tube 200 is useful in a heat exchanger in which fluid such as a gas,at relatively high temperatures, for example 1000° C., and highpressures, for example 1500 psi, flow. These elevated pressures tend tocreate tensile stresses in the joint 204. These tensile stresses tend tocause a shearing action between the fi&:ting 202 and the tube 200. Thecombination of materials 204 as discussed in connection with the joint12 of FIG. 1, resists such shearing action and provides a hightemperature, high stress structural joint not previously available inprior art structures.

It should be understood that the term member and element as employed inthe claims refers to either a structural element such as fitting 202 orto a relatively thin layer such layer 26, in FIGS. 2 and 1,respectively.

What is claimed is:
 1. A high-temperature metal-ceramic structurecomprising:a silicon based ceramic member from which silicon on onesurface is substantially removed to create a porous surface; a layer ofglass secured to said ceramic member, said layer of glass including aportion thereof in said porous surface; a layer of refractory metalbonded to said layer of glass; and a metal member bonded to said layerof refractory metal, said layer of glass providing a reaction barrierbetween said silicon based ceramic member and said metal member.
 2. Thestructure of claim 1 wherein said glass is selected from the groupconsisting of fused silica and borosilicate.
 3. The structure of claim 1wherein said metal member comprises a refractory metal.
 4. The structureof claim 1 wherein said glass layer has a thickness in the range ofabout 1-2 mm.
 5. The structure of claim 1 wherein the glass is selectedfrom the group consisting of borosilicate and quartz and the metalmember is selected from the group consisting of tungsten, molybdenum,titanium and tantalium.
 6. The structure of claim 1 wherein thesubstrate is a tube and said member is adapted to secure the tube to asupport.
 7. A composite structure comprising:a silicon based ceramicsubstrate; a silicon based ceramic layer bonded to the silicon basedceramic substrate, the silicon based ceramic layer having interstices soas to be porous relative to the substrate; a layer of glass secured tothe substrate via the interstices of the ceramic layer and having asoftening temperature of at least about 750° C.; and a refractory metalmember bonded to the layer of glass wherein the glass serves as achemical reaction barrier to preclude reactions between the member andsubstrate which reactions tend to weaken the bond of the member to thesubstrate.
 8. The structure of claim 7 wherein the glass is selectedfrom the group consisting of fused silica and borosilica.
 9. Thestructure of claim 1 wherein said ceramic member, said layer of glassand said metal member each have coefficient of thermal expansion (CTE)values sufficiently close to preclude stress failure of the structuredue to thermal cycling.
 10. The structure of claim 1 wherein saidceramic member comprises a silicon based ceramic substrate and a siliconbased ceramic layer bonded to said substrate, said ceramic layer havinginterstices so as to be porous relative to the substrate.
 11. Thestructure of claim 7 wherein said interstices of said silicon basedceramic layer are formed by silicon removed from said silicon basedceramic layer.